HTP Propulsion

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alfaro

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Hello all!

I must apologise beforehand for my lack of knowledge of much of the chemical science underlying the usage of HTP.
I also apologise if this is not the correct forum to post this question.

I have a question/idea centred around the use of a normal HTP rocket engine, and a possible way to increase its thrust, by adding a second stage to the HTP reaction.
I have been unable to find any resource online to help me understand if this is feasible or not, or if it would work at all.

The second stage would use the steam and heat generated by the first reaction, and a suitable catalyst pack - like a Zinc mesh, to further split the water steam into Hydrogen and Oxygen gases (thermochemical water splitting), which would (if the resulting gas temperature is above the auto-ignition temperature ~536ºC) ignite and expand further, generating thrust.

The reaction of Zinc and steam is a simple one to simulate - there are simple high-school level experiments using Zinc shavings and water soaked cotton inside a test tube that, when heated, generate hydrogen - example video

In an normal HTP engine, at 85% or 90% concentration, it is pressure fed by into a reaction chamber and dissociates in contact with the silver catalyst, releasing hot water steam (in excess of 600°C) and Oxygen gas (what is not trapped by the catalyst, as silver oxide).
The hot steam would then react with Zinc, resulting in Zinc Oxide and Hydrogen gas - the Zinc–zinc oxide cycle that works above around 427°C.

The Hydrogen and Oxygen gases would then auto-ignite due to their temperature.

If this added catalytic step works, it could be used in a dual catalytical cycle HTP Monopropellant Hypergolic rocket engine - it would not be as efficient and powerful as many others, but it would be simple and cheap to build.

I understand that there probably are a lot of missteps and misunderstandings of the underlying chemistry in my idea - namely with the obvious fact that such a simple setup would already have been tested and documented if it were to work.
Am I clearly missing something obvious, or I can/should try and develop my idea further into a simple test setup?

Thank you all for your time!
 

G_T

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Energy absorbed splitting oxygen from hydrogen. Temperature drops. Energy generated combining oxyten with hydrogen. Temperature goes back up. At absolute best, break-even in a system. Except, you have the weight of the Zn catalyst bed and you probably need a lot of it to have sufficient reaction rate.

Instead, burn something else in the steam + O2 mix, and even the steam can be an oxidizer. That's at least a gain. An HTP hybrid can be an example.

Gerald
 

aerostadt

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Energy absorbed splitting oxygen from hydrogen. Temperature drops. Energy generated combining oxyten with hydrogen. Temperature goes back up. At absolute best, break-even in a system. Except, you have the weight of the Zn catalyst bed and you probably need a lot of it to have sufficient reaction rate.

Instead, burn something else in the steam + O2 mix, and even the steam can be an oxidizer. That's at least a gain. An HTP hybrid can be an example.

Gerald
It's really the Law of Conservation of Energy: You don't get something for nothing.
 

rocket_troy

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It's really the Law of Conservation of Energy: You don't get something for nothing.
That's *not* what the OP was describing. He wasn't asking about any zero sum thermally decompose steam and re-react the hydrogen and oxygen. *Both* the decomposition of the HTP and the Zinc+Steam reaction would be *exothermic*. In fact, all the reactions mentioned would be exothermic.
The problem is, as I mentioned, filtering the initial oxygen products from the steam+zinc reaction and the secondary issue of zinc oxide being a likely condensed phase product thereby not really contributing much kinetic energy itself - only thermal input.

TP
 

alfaro

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That's *not* what the OP was describing. He wasn't asking about any zero sum thermally decompose steam and re-react the hydrogen and oxygen. *Both* the decomposition of the HTP and the Zinc+Steam reaction would be *exothermic*. In fact, all the reactions mentioned would be exothermic.
The problem is, as I mentioned, filtering the initial oxygen products from the steam+zinc reaction and the secondary issue of zinc oxide being a likely condensed phase product thereby not really contributing much kinetic energy itself - only thermal input.

TP
The produced Zinc oxide would (partially) dissociate back to Zinc (it occurs at above 1,900 °C) with the heat from the oxygen-hydrogen combustion - this is where the oxygen from the HTP dissociation comes in play.
The same actually happens with the HTP dissociation with Silver mesh, part of the resulting silver oxide reverts to silver with the reaction heat.
The weight of the Zinc mesh needed is indeed a great point, one that would need some kind of area maximization technique on the mesh creation.
 

rocket_troy

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The produced Zinc oxide would (partially) dissociate back to Zinc (it occurs at above 1,900 °C) with the heat from the oxygen-hydrogen combustion
So, as well as filtering the initial oxygen products from the HTP decomposition, you somehow have to filter the heat from the subsequent reactions to allow the zinc oxide reaction to happen in the 1st place.

TP
 

Charles_McG

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The zinc + steam reaction is endothermic. That's why you have to heat the sample in the linked experiment. It has to be, or the h2 + o2 to steam couldn't be exothermic. At best it's neutral, but if done in stages, more likely to end up with an overall loss due to entropy.
 

rocket_troy

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ummm. no. That's not why you heat the sample. The decomposition of HTP is exothermic as is the decomposition of N2O but you need to either heat them or expose them to a catalyst to *activate* the exothermic process. The fuel in your car needs to be heated for a similar reason to react with the oxygen for engine combustion.

You need to either heat the Zinc or water to *activate* the reaction.

Zn + 2H2O(l) = Zn(OH)2 + H2(g)

Change in Free Energy: ΔG(20C) = -81.5kJ (negative, so the reaction runs)

Change in Enthalpy: ΔH(20C) = -73.5kJ (negative, so the reaction is exothermic)


Should be

Zn +H2O=ZnO + H2 ΔH500 ◦C=−110.3 kJ (ie. Exothermic)

TP
 
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Charles_McG

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Ok - it's not the reaction I thought. That's not water splitting. But then the Zinc is not a catalyst, it's a reagent, a heavy one that's not taking part in the exhaust mass - or at least I think it's unlikely to. You're just oxidizing the zinc.
 

rocket_troy

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Well, I posted the wrong reaction. I posted a liquid water reaction and not a steam + Zinc which I think produces Zinc Oxide.

(which has been fixed now - same result)

TP
 
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rocket_troy

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ut then the Zinc is not a catalyst, it's a reagent, a heavy one that's not taking part in the exhaust mass - or at least I think it's unlikely to. You're just oxidizing the zinc.
Which is kinda the point I made above. That in of itself is not the showstopper. It's the other "challenges" I raised.

Addendum: and yes, if you wanted to strip the oxygen from the zinc after the steam reaction, that game is obviously zero-sum.

TP
 
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alfaro

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The zinc + steam reaction is endothermic. That's why you have to heat the sample in the linked experiment. It has to be, or the h2 + o2 to steam couldn't be exothermic. At best it's neutral, but if done in stages, more likely to end up with an overall loss due to entropy.
The heat necessary for the second Zinc/Steam reaction, comes from the first reaction. Zinc oxyde results from this reaction, along with releasing hydrogen.
The oxygen from the first reaction, the part that would not react with Zinc, would/could ignite with this hydrogen, generating heat that would/could regenerate the zinc oxyde, along with expanding and generating thrust.
A very many assumptions I know, all of it needs to be thoroughly tested. I just don't really know if it's worth testing, where the obvious huge mistake is.
 

Charles_McG

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Here's a thought - if the nozzle throat were made from pressed powdered Zn, then you might get a contribution in the sense you are looking for as it eroded. But I think if you run the numbers backwards, the moles of Zn consumed are going to be very small compared to the number of moles of H2O2 going in above it.
 

aerostadt

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I know that in high temperature combustion, you can include the particulates in the exhaust and average them into the molecular weight and the specific heat ratio to get predictions for performance. It is done all the time. I think running steam oxygen through a zinc mesh (maybe magnesium might be better) or grid in a rocket motor would require a new kind of engineering, if it is even possible. IMHO simpler ideas have been done. About ten years ago researchers fired a solid fuel rocket with micro-powered aluminum frozen in ice(water). It worked and had a respectable specific impulse around 300 seconds. For starters, if you have the NASA-Lewis thermo-chemical computer code (or something comparable, if there is such a thing) you could run the chemical reactions and see what kind of specific impulse you get.
 

Charles_McG

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That's a good point that aerostadt re-iterates - even if you can get the net reaction you want (H2O2 + Zn -> H2O+ ZnO) and get a little more energy, that high MW product in the exhaust is going to carry momentum per mole at lower velocity - not good for specific impulse.

On a completely whimsical note, I've considered a solid-liquid hybrid based entirely on winery chemistry: Peracetic acid and potassium metabisulfite. Peracetic acid is essentially H2O2 and acetic acid - about 25% peroxide and 30% vinegar. Not exactly high test - but higher than consumer grades of H2O2. It reacts pretty violently with the sulfite (K2S2O5). I haven't worked out the complete reaction, but figured the CH3COOH is oxidizable.
 

rocket_troy

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For starters, if you have the NASA-Lewis thermo-chemical computer code (or something comparable, if there is such a thing) you could run the chemical reactions and see what kind of specific impulse you get.
The potential problem there could be the Zn. All those tools pretty much rely on the thermochem data within the JANAF tables - specific heat capacities, enthalpies, entropies through polynomial conversion databases for all potential product and intermediate species covering a broad temperature range. The issue is, there's actually not much in the JANAF databases themselves (the source of the data) for Zn ie for its oxides or sulphides or whatever. It's like a neglected element for thermochem work.

TP
 
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Charles_McG

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The heat necessary for the second Zinc/Steam reaction, comes from the first reaction. Zinc oxyde results from this reaction, along with releasing hydrogen.
The oxygen from the first reaction, the part that would not react with Zinc, would/could ignite with this hydrogen, generating heat that would/could regenerate the zinc oxyde, along with expanding and generating thrust.
A very many assumptions I know, all of it needs to be thoroughly tested. I just don't really know if it's worth testing, where the obvious huge mistake is.
I misunderstood the Zinc reaction when I said what you quoted. But in any case, I don't think you can play with energy/enthalpy the way you just described above. In order to 'regenerate the zinc', you'd have to reverse the reaction, and that definitely nets you back out. The temperature might be there, but the heat movement reverses any gains.

I think I wrote the net reaction correctly above:
2 H2O2 -> 2 H2O + O2
2 Zn + 2 H2O -> 2 H2 + 2 ZnO
O2 + 2 H2 -> 2 H2O
or twice
H2O2 + Zn -> H2O + ZnO.

If you try to recover the 'burned' Zn, you lose it for consuming the O2.
 
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aerostadt

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rocket_troy

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Indeed, John Wickman also tried to sell similar concepts using CO2 + Mg for Mars type environments. Many different ideas and concepts and many wheels re-invented and here we are, back with grandma's favourate recipes.

TP
 

alfaro

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I really must thank you all for your replies, I now understand the logical error in my proposal.

Although the Silver used in the HTP dissociation is a catalyst, with non-zero but minimal consumption/losses over time, the Zinc used in the second H2O splitting is a reagent, that is consumed in the reaction, and would be quite hard/near impossible to self-regenerate.

It could still work for a short time, and some bit of hydrogen-oxygen combustion would occur in the output, but I doubt it would be any better, most probably way worse, than the force output of a simple HTP dissociation.
 
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